Field of the Invention
The present invention relates to an antenna device, and particularly to an antenna device that is suitable for NFC (Near Field Communication). The present invention also relates to a portable electronic device in which such an antenna device is mounted.
Description of Related Art
In recent years, in portable electronic devices such as smartphones, a system of RFID (Radio Frequency Identification) is mounted. As a communication means for the system, an antenna is mounted to perform near field communication with a reader/writer or the like.
Meanwhile, in the portable electronic device, in order to protect an internal circuit from external noise and prevent unnecessary radiation of noise generated inside the device, a metal shield is provided. In particular, in order to make the body thinner, lighter, and more resistant to shock such as when the body is dropped, and to improve the design and other factors, the housing of a recent portable electronic device itself has increasingly been made of metal instead of resin, with the housing doubling as a metal shield. However, in general, the metal shield blocks radio waves. When an antenna is provided, the antenna needs to be placed in such a way as not to overlap with the metal shield. If the metal shield is provided across a wide range, how to dispose the antenna becomes a problem.
To solve the above problem, for example, in an antenna device disclosed in Japanese Patent Application Laid-open No. 4,687,832, Japanese Patent Application Laid-Open No. 2002-111363, or Japanese Patent Application Laid-Open No. 2013-162195, an opening is formed in a conductor layer, and a slit is formed in such a way as to connect the opening to an outer edge; an antenna coil is disposed in such a way that an inner diameter portion overlaps with the opening. In the antenna device, current flows through the conductor layer in such a way as to block a magnetic field generated by a flow of current through a coil conductor; and the current flows around the slit that flows around the opening of the conductor layer, and the current also flows around the conductor layer due to the edge effect. As a result, a magnetic field is generated from the conductor layer, and the conductor layer makes a large loop of the magnetic flux, resulting in a longer communication distance between the antenna device and an antenna that the antenna device is communicating with. That is, the conductor layer functions as an accelerator that helps to increase the antenna coil's communication distance.
However, in the above-described conventional antenna device, the opening and slit need to be formed in the conductor layer. Therefore, the problem is that this configuration restricts the layout of the antenna coil. For example, if the opening cannot be formed at a desired position due to design constraints, or if the slit is not allowed to be formed even as the opening can be formed, the problem is that the antenna device cannot be configured. A similar problem arises when an opening used to expose a lens of a camera module cannot be used as an opening for an antenna coil.
An object of the present invention is to provide an antenna device that can increase a communication distance of an antenna coil even when an opening is not provided in a conductor layer provided in a portable electronic device. Further, an object of the present invention is to provide a high-performance portable electronic device that is formed by using such an antenna device.